1. Field of the Invention
The present invention relates to a semiconductor device having a function to control a current supplied to a load by a transistor, and to a display device including a pixel formed with a current-drive display element of which luminance is changed by a signal, a pixel formed with a voltage-drive display element of which luminance is changed by a voltage, and a signal line driver circuit and a scan line driver circuit thereof. The present invention also relates to a method for driving the same. The present invention further relates to an electronic device including the display device in a display portion.
2. Description of the Related Art
In recent years, a so-called self-luminous display device in which a pixel is formed using a display element such as a light emitting diode (LED) has attracted attention. As a display element used for such a self-luminous display device, an organic light emitting diode (also referred to as an OLED, an organic EL element, an electroluminescent (EL) element, or the like) has attracted attention, and has been used for an EL display and the like. Since a display element such as an OLED is of self-luminous type, it has advantages such as higher pixel visibility, no backlight required, and higher response speed compared to a liquid crystal display. Note that the luminance of the display element is controlled by the value of current flowing therethrough.
As a method for driving such a display device to express a gray scale, there are an analog gray scale method and a digital gray scale method. The analog gray scale method includes a method to control the light emission intensity of a display element in an analog manner and a method to control the light emission time of a display element in an analog manner. As the analog gray scale method, the method to control the light emission intensity of a display element in an analog manner is often used. However, the method to control the light emission intensity in an analog manner is easily affected by variations in characteristics of a thin film transistor (hereinafter also referred to as a TFT) of each pixel, which causes variations also in luminance of each pixel. On the other hand, in the digital gray scale method, a display element is turned on/off by control in a digital manner to express a gray scale. In the case of the digital gray scale method, the uniformity of luminance of each pixel is excellent. However, there are only two states, that is, a light emitting state and a non-light emitting state, so that only two gray scale levels can be expressed. Therefore, multiple level gray scale display is attempted by using another method in combination. As a technique for multiple level gray scale display, there are an area gray scale method in which light emission area of a pixel is weighted and selected to perform gray scale display and a time gray scale method in which light emission time is weighted and selected to perform gray scale display. In the case of the digital gray scale method, the time gray scale method, which is also suitable to obtain higher definition, is often used.    [Patent Reference 1] Japanese Patent Publication No. 2784615
Here, improvement in definition can be achieved by using the time gray scale method in the digital gray scale method. However, as improvement in definition proceeds, the number of pixels is increased. Therefore, the number of pixels to which a signal is written is also increased.
In addition, the number of subframes needs to be increased to perform high level gray scale display. Therefore, the number of times signal writing to a pixel is carried out is increased.
Thus, with improvements in definition and level of gray scale display, the number of times charging and discharging are carried out, associated with signal writing operation, is also increased. An increase in power consumption becomes problem.